In the operation of turbine-generators in electric power plants, the electrical load supplied by the generator is dependent on the steam flow through the turbine. Typically, a plurality of governor valves function in parallel steam flow paths to control the total turbine steam flow in response to the operation of a turbine valve controller.
Each governor valve has associated with it a partial steam admission arc, i.e. an arcuate turbine region into which steam is admitted through nozzles or nozzle vanes from a nozzle block. If all of the governor valves are open, a continuous steam admission arc is formed fully about the turbine. Therefore, if all governor valves are operating in an open position the turbine is considered to be operating in a full arc mode, and if one or more governor valves are closed, the turbine is considered to be operating in a partial arc mode. Continuous arc operation is usually called single valve operation and partial arc operation is usually called sequential valve operation.
With respect to sequential valve operation, the turbine manufacturer typically defines the sequence in which the governor valves are to be operated to increase or decrease load, principally as a result of consideration of the effects of thermal and mechanical stresses on turbine life. Typically, single valve operation is used during turbine startup and/or load operation up to load levels where loss is throttling efficiency makes sequential valve operation desirable. Sequential valve operation is ordinarily employed to obtain higher heating rates at higher loads although it can be employed at lower loads and even during startup with some penalty from a rotor stress cycling standpoint.
Generally, undesirable time dependent force fields can be imposed on the rotating blades during partial arc turbine operation, and in certain cases the force field on the rotating blades can be such that a double or multiple blade shock condition exists. Highly desirable blade vibrations and blade vibratory stresses are generated during a multiple shock condition. Blade life is dependent on the blade cyclical stress history and it can especially be foreshortened by vibratory stresses associated with multiple blade shock. For this reason, double or multiple blade shock loading or other excessive cyclical blade loading is desirably avoided during turbine operation.
Generally, normal and steady and normal fluctuating drive stresses on the rotor blades are taken into account in the blade and rotor design and they therefore normally present no unusual problem. Although a time dependent temperature field can be applied to the rotating rotor blades during partial arc operation, the resulting blade thermal stresses are normally inconsequential as compared to the vibratory stresses associated with multiple shock.
Multiple blade shock can occur as a result of failure of one or more governor valves in the closed position such that other open valves produce two or more separate and partial steam admission arcs. In the double blade shock condition, the rotating rotor blades successively move through a first partial steam admission arc, a second partial arc through which no valve admitted steam flows, a third partial arc through which steam flows, a fourth partial arc through which no valve admitted steam flows, and then through the first partial arc to repeat the cycle. Each time a blade enters a partial steam admission arc, a shock occurs, i.e. a large increase occurs in the driving force on the rotor blade. During any one rotation, two blade shocks and associated blade vibratory stresses occur in the desired case. A greater number of blade shocks can occur in any one rotation according to the number of times a blade enters partial steam admission arcs or undergoes large increases in driving force during any one rotation.
In normal sequential valve operation, the manufacturer defined sequence for the governor valves is one in which the rotating blades undergo a single shock each rotation if there is at least one partial arc through which no steam is being admitted. Although single shock operation is undesirable to some degree, it is unavoidable and therefore must be tolerated during sequential valve operation.
In order to eliminate double blade shock or other undesirable blade loading which may occur in the sequential governor valve operating mode from time to time, the turbine can be protectively transferred from sequential valve operation to single valve operation to obtain a full steam admission arc. Of course, in certain cases, such as if a pair of arc displaced governor valves fail closed simultaneously, a double blade shock condition could exist even in the single valve operating mode and in that event it may not be possible to avoid the double shock without turbine shutdown.
While single valve operation can avoid excessive blade loading in the sequential mode, a transfer from sequential valve operation to single valve operation or vice versa does cause a rotor thermal stress cycle as a result of new rotor surface temperature conditions in the new mode. Therefore, protective mode transfers should not be initiated where the rotor stress cycle coat is not justifiable.
In the prior patent art such as Eggenberger U.S. Pat. No. 3,403,892, there is disclosed an electrohydraulic turbine valve control system in which analog control circuitry is provided for effecting automatic transfer between sequential valve and single valve operations with small load change while the turbine is operating on line. In the above noted copending applications there is disclosed a computer or a digital electrohydraulic turbine valve control system in which improved on-line valve transfers or valve management operations are provided. Although the prior art provides for a valve transfer capability, none of the known prior art is organized to provide rotor blade protection against undesirable blade loading and particularly against multiple shock while retaining bumpless turbine operation.
In the present application, no representation is made that any cited prior patent or other art is the best prior art nor that the interpretation placed on such art herein is the only interpretation that can be placed on that art.